function c=constel(x,fs,fb,fc)
N=length(x);
m=2*fs/fb;
n=fs/fc;
i1=m-n;
i=1;
ph0=(i1-1)*2*pi/n;
while i<=N/m
    xi=x(i1:i1+n-1);
    y=2*fft(xi)/n;
    c(i)=y(2);
    i=i+1;
    i1=i1+m;
end
if nargout<1
    cmax=max(abs(c));
    ph=(0:5:360)*pi/180;
    plot(1.414*cos(ph),1.414*sin(ph),'c');
    hold on;
    for i=1:length(c)
        ph=ph0-angle(c(i));
        a=abs(c(i))/cmax*1.414;
        plot(a*cos(ph),a*sin(ph),'r*');
    end
    plot([-1.5 1.5],[0 0],'k:',[0 0],[-1.5 1.5],'k:');
    hold off;
    axis equal;
    axis([-1.5 1.5 -1.5 1.5]);
end